The reason I say that is it would force a different plot to the movie than "bad guy about to destroy all of Gotham and Batman has to race around in the Batmobile to stop it". What's Deathstroke really going to be that he wasn't already on Arrow? What will he be that Al Ghul, the Joker and Bane/Talia weren't already?
Writing a plot around Zsasz would force the focus away from "stunt spectacular" and draw out more of the World's Greatest Detective angle. It would be a chance to highlight the type of threat a character like Zsasz could be, if he weren't horribly mishandled. His original appearance was mystery, of how he was seemingly managing to get more kills while trapped inside of Arkham. In other appearances he's thrown away pathetically by him blatantly holding hostages and confronting the police and/or Batman. Imagine a Zsasz that's a scalpel to the Joker's cream-pie bazooka. The Joker revels in the widespread chaos he's caused, flamboyantly causing mayhem and confronting Batman openly. Zsasz kills quietly, patiently, elusively. The conflict is indirect, but still personal. The chaos isn't an exploding hospital, but a surgeon killed just before a heart transplant. It's not a bomb set to destroy the city, it's a foreign diplomat killed when tensions are already high. It's exactly the wrong person killed at the worst possible time.
Imagine a Batman who hasn't even seen the killer he's chasing. A Batman who has brought down mob bosses and psychopaths, but isn't even quite sure that one person is behind the Zsasz killings. A Batman who suspects that these murders are meant to not just kill a person, but to cause further chaos with the deaths. A Batman who has been sifting through clues for months, but still can't put a face or a motive to a series of crimes that might just be getting more aimed at the Batman and his work as time goes on. Make a killer who causes Batman to not just fear for the people of Gotham, but one who causes Batman to fear the limits of his own detective skills, possibly one who makes Bruce doubt his own sanity.
Granted, such a tale of suspense is not going to happen at the hands of the current DC higher-ups, but at least Zsasz as a villain wouldn't easily lend itself to the "Batman stunt spectacular" that I'm growing a little tired of.